1. Field of the Invention
The present invention relates to a tuner circuit used for a television receiver. In particular, the present invention relates to a tuner circuit, which is adaptable for controlling a high (radio) frequency filter characteristic.
2. Description of the Related Art
In television receivers for VHF and UHF bands, a tuner circuit converts a received signal ranging from about 100 MHz to 1 GHz into an intermediate frequency signal having tens of MHz. In the process of manufacturing the tuner circuit, there is provided a pre-filter included in a front stage of the tuner circuit, that is, a received signal amplifying stage. The pass band characteristic of the pre-filter is tuned and controlled to a frequency of the received signal. In this case, the output signal frequency characteristic of the tuner circuit is observed using a network analyzer. The tuner circuit includes an intermediate frequency filter. For this reason, it is difficult to measure the pass bad characteristic of the pre-filter using a signal passed through the tuner circuit.
Japanese Patent No. 3178382 has descriptions related to the following method. According to the method, a resonator of the intermediate frequency filter in the tuner circuit is electrically short-circuited to control the characteristic of the pre-filter. By doing so, the characteristic of the intermediate frequency filter is made flat as much as possible. However, according to the foregoing method, it is impossible to fully remove an influence on the characteristic of the intermediate frequency filter. In other words, the frequency characteristic of a tuner output signal receives the influence on the characteristic of the intermediate frequency filter. In addition, the work of controlling the characteristic of the pre-filter becomes troublesome.